Wireless broadband technology offers the prospect of mobile alternatives to high speed, wired, voice and data communication systems (e.g., fiber optic or copper wire broadband transmission systems). As with conventional radio frequency (RF) devices, an important component for up-conversion (transmission) or down-conversion (reception) in microwave broadband communication devices is the local oscillator, and particularly, the voltage-controlled oscillator (VCO), that operates in the frequency range of the carrier signal. Thus, the VCO is an essential component for up/down conversion of the transmitted signal. Typical design criteria for VCO's are frequency stability, high output level, tenability, low phase noise, small packaging and low cost. Further, in order to increase the bandwidth of the transmitted RF signal, thereby increasing the rate of data transmission at which such wireless broadband communications systems can operate, VCO's in transmitters must be capable of generating very high, microwave carrier frequencies, that is, in the 10 gigahertz range and above. One example is the 28 GHz band of local multipoint distribution services (“LMDS”) systems.
Moreover, with the increasing market demand for more powerful and smaller wireless communications systems with greater bandwidth capacity, such as wireless networked portable computers, personal digital assistants (“PDA”) and other specialty communications devices, and the convergence of voice and data, there is a need for high frequency, broadband tunable VCO's that can be integrated into the microwave front-end circuits (transceivers) that are themselves integrated with digital back end circuits on a single integrated circuit (“IC”) chip.
Unfortunately, existing lumped solutions for such integrated, high frequency oscillators are inadequate. For example, while it is possible to design a tunable LC resonant tank oscillator circuit on a silicon substrate at up to 10 GHz, it becomes excessively difficult to achieve a wide tuning range and good phase noise as the frequency of operation approaches the fmax, or cut off frequency, of the transistors. This is mainly due to the trade off between the self resonance frequency and the quality factor, Q, of the integrated inductors and varactors, which is very low for operation at frequencies above the C band (i.e. above about 6.5 GHz). This trade off becomes prohibitive as the operating frequency increases.
Thus, there exists a need for a microwave voltage controlled oscillator (“VCO”) that (1) is small, i.e. capable of being designed as part of an integrated circuit (IC) package; (2) is low cost; (3) provides stable operation; and (4) is capable of wide band tuning.
The distributed oscillator has recently been considered as a possible low cost microwave VCO solution in CMOS radio frequency integrated circuits (“RFIC's”), due to its ability to operate at frequencies close to the intrinsic cutoff frequencies of the transistors. The distributed oscillator originates from the distributed amplifier, which has been studied for many years. For example, Skvor, et al. proposed to build a VCO by operating a distributed amplifier in the reverse gain mode, using the output from the idle drain load as the feedback output. See, “Novel Decade Electronically Tunable Microwave Oscillator based on the Distributed Amplifier,” Electronics Letters. vol. 28, no. 17, pp. 1647 1648, August 1992. Further, a 4 GHz, distributed oscillator was assertedly demonstrated using discrete pHEMTs and microstrip lines on a printed circuit board (PCB). Divina L., Skvor Z., “The Distributed Oscillator at 4 GHz.” IEEE Trans. MIT, vol. 46, no. 12, pp. 2240 2243, December 1998. Another group recently assertedly showed an integrated (with off chip termination and bias) distributed oscillator operating at 17 GHz without any tuning capability using 0.18 mm CMOS technology. The forward gain mode instead of reverse gain mode was used and assertedly demonstrated that CMOS is viable for oscillator applications at microwave frequencies. See Kleveland B., et al., “Monolithic CMOS Distributed Amplifier and Oscillator,” IEEE Int. Solid State Circ. Conf., Paper MP 4.3, February 1999.
Despite these apparent advances, tuning remains a problem since distributed VCO's (“DVCO's”) are used at frequencies close to the device fT, where there is not enough gain to lose in tuning circuitry. Consequently, the addition of extra integrated varactors with low Q is not a favorable option due to their high loss which further deteriorates with frequency. Nor can the reverse mode tuning scheme described in the above referenced Skvor et al. article be used due to the limited transistor gain in CMOS technologies. Therefore, a new tuning scheme must be devised.
Accordingly, it should be appreciated that there exists a definite need for a sufficiently tunable, operatively stable, and relatively low cost and integrated DVCO.